The "classic" Master Cleanse (aka "Strict Burroughs") original recipe indicates that the laxative detox tea made from the herb senna is required daily, morning and night.
I for one would only do it in the evening, right before bed, when I am so tired I can't stay up anymore so that I know by the time the laxative is causing rumbles I am sound asleep or just about to wake up. I also only do it once because I know I am going to do the salt water "internal bathing" in the morning:
The salt water flush is mentioned as "Another cleansing aid: internal salt water bathing" which actually gets more print that the laxatives do. Stanley further explains:
As it is necessary to bathe the outside of our bodies, so it is with the inside. Do not take enemas or colonics at any time during the cleansing diet or afterwards. They are unnecessary and can be extremely harmful. There is a much superior method of cleansing the colonic tract without the harmful effects of customary colonics and enemas. This method will cleanse the entire digestive tract while the colonics and enemas will only reach the colon or a small part of it. Colonics can be expensive while our salt-water method is not.
You can read more in Stanley's book here.
Many people do both, but it is not usually necessary. If you do the SWF (Salt Water Flush) properly, you WILL have a movement. Sometimes people get the mixture of salt/water wrong, and their body absorbs the salt. If the SWF is not working no matter what you do, just make sure you are doing the laxative tea. I do, however, think the SWF is much more beneficial, since all that salt water bathes your intestines and helps clear things out.
My preferred approach is to altenate each day. Laxative only every other day. Salt water flush on the other days.